1. Field of Invention
This invention relates to an apparatus for deforming thermoplastic sheet material.
2. Description of Related Art
Certain thermoplastic materials referred to as engineering thermoplastics, have relatively high strength, high impact resistance, are tough, and have high temperature and load bearing capabilities making them particularly useful in certain applications. For example, thermoplastics are achieving more widespread use in automotive applications including bumpers and other component parts requiring high strength. It would also be desirable to form large body panels of automobiles out of sheet material plastics. However, present processes for producing panels from thermoplastic materials have certain drawbacks.
Presently, notwithstanding the process of the aforementioned patent, two general types of processes are used to form thermoplastics into large parts: injection molding and sheet stamping. Each of these processes has substantial disadvantages, particularly with respect to the forming of large thin parts. As disclosed in the aforementioned patent, injection molding processes for thermoplastics are typically limited to parts less than approximately 600 square inches of surface area. This limitation is imposed by large clamping forces which must be applied to mold dies to counteract large forces generated by high injection pressures acting on large part areas.
Sheet stamping processes are also limited in their application to thermoplastics. One aspect of the problem is lack of current knowledge regarding large deformation behavior of thermoplastic materials. A further problem is that sheet stamping processes require matched dies to be compressively directed against one another to squeeze under compressive load the sheet material therebetween. The simultaneous contact of two dies for compressively squeezing the material therebetween, when deep drawing portions of the material is required, causes relatively large localized tensile loads on the material which tends to tear the thermoplastic material.
The stretching of the material results from the fact that points of the material surrounding deep drawn concave or convex areas are simultaneously clamped while the portions of the material which form the convex or concave regions are subject to relatively large elongations which thins the material. This stretching action may tear the thermoplastics materials. Additionally, the stamping process, because it requires formation of the entire sheet material simultaneously, requires relatively large stamping load forces. Such machines are costly. The present inventors recognize a need for an apparatus and a process for forming thermoplastic material which is cost effective and which is practical for forming large sheet component parts without tearing the material.
In the aforementioned U.S. Pat. No. 4,818,460, a procedure is disclosed for roll-forming thermoplastic sheet material. However, the present inventors recognize that the roller employed in the disclosed procedure, being pliable and deforming generally to the shape of the lower die against which the sheet material is formed does not provide sufficient detail to the surface of the formed sheet material engaged by the roller for certain implementations. In certain instances, it is desirable that the sheet material be deformed with sharp, clearly defined detail on both surfaces. Such sharp detail normally is produced by two dies in a stamping operation. Therefore, the present invention is directed toward providing sufficient detail to the themoplastic material as normally produced by two mating dies as employed in the stamping process while alleviating the tearing and large machine force problems of prior art stamping machines.